The human anatomy includes many types of tissues that can either voluntarily or involuntarily, perform certain functions. After disease, injury, or natural defects, certain tissues may no longer operate within general anatomical norms. For example, organs such as the heart may begin to experience certain failures or deficiencies. Some of these failures or deficiencies can be diagnosed, corrected or treated with an implantable medical device (IMD). For example, an implanted IMD may detect an arrhythmia, such as ventricular fibrillation, and deliver one or more electrical pulses to stop the arrhythmia and allow the heart to reestablish a normal sinus rhythm.
Examples of such IMDs include subcutaneous implantable cardioverter/defibrillator (SICD) systems that provide synchronous cardioversion shocks and/or asynchronous defibrillation shocks and subcutaneous pacemaker/cardioverter/defibrillator (SPCD) systems that provide additional staged therapies of anti-tachyarrhythmia pacing, synchronous cardioversion shocks and asynchronous defibrillation shocks. In general, the IMDs deliver a first pulse at a first energy level upon detecting an arrhythmia and, if the arrhythmia is not stopped, deliver additional pulses at increasing energy levels until the arrhythmia is stopped or the programmed progression of pulses has been exhausted.
Typically, threshold testing is performed to evaluate the effectiveness of an IMD in ending episodes of arrhythmia. For example, the energy levels or waveforms of pulses delivered by the IMD, the sensitivity of the IMD to detect ventricular fibrillation, or the position of the electrodes used to deliver the pulses, can be configured as necessary to assure the effectiveness of the IMD. The threshold testing may be performed during the implantation process, during subsequent follow-up sessions and/or during the automatic configuration sessions initiated by the IMD. One method of testing an IMD's capability to reliably defibrillate the heart involves induction of an episode of an arrhythmia in the patient's heart, and then allowing the IMD to detect and terminate the induced arrhythmia. The IMD itself has the capability of inducing arrhythmia during the threshold testing procedures.
The IMD induces an arrhythmia by delivering a pulse during the period of vulnerability within a cardiac cycle, e.g., during or near the T-wave, delivering a high frequency pulse train, delivering direct current, or other known methods for inducing the fibrillation. The clinician may program the stimulation parameters for the induction attempt, such as the timing, amplitude, or other characteristics of a T-wave shock. If the induction attempt fails, the new stimulation parameters are used for another induction attempt.
When an induction attempt succeeds, the IMD can fail to detect the arrhythmia, or fail to stop the arrhythmia. In such cases, the detection algorithm or the pulse progression must be modified. The process repeats until successful arrhythmia induction, detection, and defibrillation occur such that the effectiveness of the IMD is confirmed.
The process of confirming the effectiveness of an IMD can be time and resource consuming both for the clinician and for the IMD. For example, a clinician programs the IMD to execute an initial arrhythmia detection algorithm, and programs an initial progression of pulses to be delivered in response to a detected arrhythmia. The clinician then programs the IMD to induce the heart to fibrillate, so that the programmed detection algorithm and pulse progression can be tested. During automatic configuration sessions, the device may also consume a large amount of the internally stored energy to perform those functions. Accordingly, there remains a need for improved circuits and methods for therapy delivery.